Recent work (see Significance) has led to my proposing a fundamental change in the sliding filament model of striated muscle. This change is necessary to account for the long-range elastic properties of striated muscle. In the proposed model, a third filament is postulated to elastically connect each thick filament to the adjacent Z lines of the sarcomere and, further, to function as the core of the thick filament. This application proposes further mechanical, structural, and biochemical work designed to further test and extend the third filament model. Studies of single, skinned fibers of frog, human, rabbit, cricket, and water bug striated muscle will seek to correlate mechanical and ultrastructural behavior of fibers stretched on the force-length transducer, fixed in situ, and then thin-sectioned for electron microscopy. Attention will focus on differences in, and similarities between, muscles which differ greatly in passive stiffness. Extraction of fibers, fibrils, and filaments will be studied by SDS-PAGE electrophoresis, EM, and transducer seeking to correlate changes in mechanical properties with changes in the contents of extracted structures. X-ray diffraction analysis of third filament sub-structure will also be attempted. Fibers dissected from biopsies from patients suffering myopathies will be studied with the transducer, EM, and gels to seek an effect of degenerative disease on the long-range elasticity mechanism. The significance of the proposed work is that a newly-recognized myofibrillar structure provides a new basis for seeking an understanding of unresolved questions about muscle function, structure, and chemistry. This work promises a better framework to understand the differentiation, growth, maintenance, hypertrophy, and atrophy of striated muscles in animals and man.